Interpretive Summary: Powdery mildew disease caused by a fungus, has become a major problem on watermelons in recent years. The best way to control the disease would be by growing varieties that are genetically resistant to the disease, but no source of such resistance has been identified. Therefore, we performed artificial inoculations on 266 watermelon accessions that have been collected by the USDA from all over the world to search for resistance. We found a few of these had moderate resistance, which may be useful to help control this disease, once this resistance is incorporated into commercial varieties.

Technical Abstract:
Two hundred sixty-six Citrullus lanatus (Thumb.) Matsum. and Nakai accessions (Plant Introductions and named cultivars), were tested against a race 2 Sphaerotheca fuliginea (Schlechtend.: Fr.) Pollacci to evaluate for resistance to powdery mildew disease. Growth room grown seedlings were artifically inoculated with conidia from watermelon hostleaves at two day intervals from the appearance of the first true leaf until test results were taken, when the second true leaf was fully expanded. Plants were evaluated on a 1-9 scale of increasing disease severity. Disease indices (DI's) were calculated as weighted averages for each entry. All genotypes with resistant plants (powdery mildew rating 1-3) were reevaluated in a replicated test of 3 reps of 10 plants each. Disease indices were again calculated. Twenty-two Plant Introductions (PI's)and one named variety displayed intermediate resistance to powdery mildew in the replicated test with DI's ranging from 5.0-6.0